Means comprising a source of coherent radiant energy for the production of ions for mass spectrometry



Dec. 27, 1966 L. JENCKEL 3,294,970

MEANS COMPRISING A SOURCE OF COHERENT RADIANT ENERGY FOR THE PRODUCTION OF IONS FOR MASS SPECTROMETRY Filed 001.. 24, 1962 United States Patent 3,294,970 MEANS CQMPRISENG A SSURCE F COHERENT RADIANT ENERGY FOR THE PRODUCTION 9F IGNS FOR MASS SiECTRGMETRY Ludolf Jenckel, Muhlenthal 15, Bremen-St. Magnus, Germany Filed Oct. 24, 1962, Ser. No. 233,2tl4 Claims priority, application Germany, October 26, 1961, A 38,667 6 Elaims. (Cl. 25041.9)

The present invention relates to a method and means for production of ions, in particular for mass spectrometry. In the known ion sources there are parts which have to be brought to a higher temperature than other adjacent parts. Such parts are i.e. the cathode for the emission of electrons when it is used in an ion source for ionization by electron-collision, and the evaporation furnace in which solid substances are brought into a gaseous state prior to being ionized by electron collision. Furthermore, it is required that in a so-called thermal ion source the ionization filament and when having a double filament ion source of this type the evaporating filament, too, should be brought to a higher state of temperature mainly when ionizing suitable non-organic compounds.

In general it is desirable to localize the heat on places where it is most needed. It is, however, undersirable if the heat spreads to adjacent parts which may cause the escape of absorbed gases and impurities, causing the formation of high background ion currents which in some cases makes the detection of components impossible, as those appear only in minor concentration within the sample to be tested.

The place to be brought into a higher state of temperature is further dictated by the fact that the location of the ionization zone is influenced by the high temperature of that place. This is the case, i.e., at the cathode in an electron-collision ion source. The ionization zone and the most favourable conditions of adjustment for the transmission of ions into the analyzing tube of the mass spectrometer is dependent on the place at the filament where the electrons are being emitted. Strong displacements of the ionization zone have already been noted in the use of a spark discharge ion source. Great fluctuations in the measured ion currents were caused by the until now unavoidable spreading of sparks over the discharge electrodes.

In order to avoid the above encountered problems in an ion gun, it is an object of the present invention to use an irradiation of strongly collimated rays of high intensity, such as can be obtained from an optical laser for heating the samples or electrodes in the ion source. This offers the advantages of restricting the irradiation to a small prearranged area on the sample or electrode and adjusting the duration of irradiation. As a result, the background ion currents as well as undesired fiuctuations of ion currents can be kept at minimum level.

The heating of the sample or electrode according to the method of the present invention has the operational and constructional advantages that the heat source is arranged outside the vacuum chamber of the ion source. Consequently, the heating can be localized on a restricted area which means further that no heat producing parts are contained within the ion source of the mass spectrometer, since the heat radiation source is arranged outside the vacuum chamber of the ion source, which also represents a unit completely separated from the mass spectrometer itself. Since there is no fixed connection required between the mass spectrometer and the source for irradiation, the latter can also be used for other purposes, i.e. for the irradiation of other ion sources, etc.

For the production of ions by spark discharge occurring between two electrodes, the spreading of sparks can be avoided by the irradiation of one or both of the electrodes which causes on the area of radiation incidence the generation of locally restricted heat which in turn increases the vapour pressure, thereby restricting spark discharge to these areas. It offers an important advantage in that the increased vapour pressure will already at lower voltages and at lower frequencies cause the ignition of discharge which until now was only possible by the application of DC. voltages. When applying lower voltages, the sample molecules will notbe dissociated to great extent in the path of the sparks, and so the low frequencies will not cause disturbance to the sensitive detecting instruments in the measurement of ion currents. During irradiation from laser rays which effects the momentary increase of vapour pressure in certain circumstances, the same effect can also be achieved by applying ultra-violet radiation to one or both of the electrodes, thereby causing the release of photoelectrons by the momentary increase of charge carrier density. Naturally, an increased effect will only be reached through the simultaneous ap plication of both irradiation sources.

With an apparatus used for the irradiationof an electrode through laser rays, the spark discharge ion source can also be used as normal electron-collision ion source. If the energy density of the rays is high enough to cause the emission of thermal-electrons from the electrode being irradiated, the irradiated electrode can be used in the place of the cathode and the other electrode in the place of the anode (ion collector). In this manner the troublesome exchange of different ion sources can be avoided when, e.g., one wishes to change from spark discharge ion source to an electron-collision ion source.

In the drawing there are illustrated by way of example various embodiments of an ion source to accomplish the method of the invention.

FIG. 1 shows an evaporating furnace ion source for the ionization of solid substances.

FIG. 2 shows a spark discharge ion source, and

FIG. 3 an ion source for the ionization of gaseous substances.

In the illustrated embodiments shown in the drawings the ion source is arranged in a vacuum chamber 1 of a mass spectrometer head part 2 to which the analyzing tube 3 of the mass spectrometer is being secured vacuum tight in the usual manner. The ions which are produced in the ion source are being directed in the usual way in the form of strongly focused beam a through an extracting electrode system 19, this latter being at suitable voltage potential, out of chamber 1 through an entry slot 4 into the analysing tube 3 of the mass spectrometer.

For the ionisation of solid substances according to FIG. 1 there is positioned within the vacuum chamber 1 a casing 5 which is enclosing the actual ionization space. In casing 5 there is provided a device (not shown) for holding the solid or liquid samples b which are to be ionized.

The ionization of the solid or liquid samples is performed in the usual manner by evaporating of the sample substance and following ionization by electron-collision between a heated cathode 6 and an electron collector 7 along the path 0. Openings 8 and 9 are provided on casing 5 being in alignment with the flowing electron beam on path c.

The ions formed by evaporating the sample prior to the electron-collision ionization are sucked out of easing 5 through a window 11 and by a system of electrodes 10 are focused into a narrow bundle of rays directed through an entry slot 4 into the analysing tube 3 of the mass spectrometer.

The vaporisation of the sample is caused by the irradiation from an optical laser 12 having means to adjust the duration of radiation being arranged outside the head part 2 of the mass spectrometer. The laser beam d enters the vacuum chamber 1 through a window provided with a lens 13 and reaches the sample b through an opening 14 in casing 5. The opening 14 is aligned with the optical axis of lens 13, the optical axis of which in turn is further aligned with the axis of laser beam d.

By the application of laser beam d high thermal energy is concentrated through lens 13 on the sample which is sufiicient to melt and/ or evaporate instantaneously even refractory substances.

From the above description it can readily be appreciated that non-heat producing parts are housed within the vacuum chamber 1, thereby greatly contributing to the constructional simplicity of the ion source.

In the spark discharge ion source as shown in FIG. 2 spark discharge 1 is produced between two electrodes 15 and 16, one of which, i.e., 16 consists of the sample to be ionized. Similar to the embodiment described in connection with FIG. 1 the ions formed by the spark discharge 1 are focused by an electrode system a into a narrow collimated beam of rays a directed through entry slot 4 into the analysing tube 3 of the mass spectrometer.

For the production of sparks f the electrodes and 16 are connected to a high voltage source 17. This high voltage source may comprise a high frequency generator, low frequency generator or DC. voltage source, too.

As shown in FIG. 2 a laser 12 is arranged outside the head part 2 of the mass spectrometer causing the emission of strongly focused rays d being directed through lens 13 onto oppositely lying electrode 16. In actual operation the electrode 16 may be heated up to such a degree that it starts to evaporate, causing the intensive emission of thermal electrons analogous to the emission of electrons from a heated tungsten filament. The thermal electrons are accelerated in the direction of the adjacent electrode 15, causing essentially an electron collision ionization with great stability within the area where the sparks f are being discharged. An advantage of the new method is seen in that the area in which ionization of the samples takes place is small when compared with ion guns of conventional nature. A further advantage is that the development of heat is concentrated on a comparatively small area, thereby greatly reducing the overall heat-spread around the electrodes; it is also noticeable that the vapour pressure is increased sufficiently high and leads to the ignition of sparks which could not be obtained at normal temperature by applying the ignition voltage.

The high voltage source 17 and laser 12 may be coupled through cable means 18, in order to time the voltage and laser pulses so that without operating the laser, the high voltage source is disabled for a required duration and so maintains great stability of the process of ionization.

In FIG. 3 there is shown an ion source for the ionization of gaseous substances. Contrary to heating a cathode or electron-emitter in the known ion sources, the present invention provides novel means by irradiating the cathode '6 from a laser 12 causing a locally restricted heating of the cathode by laser rays d.

Due to the comparatively long wave characteristics of the laser rays it is advantageous to use an additional irradiation source, i.e., an ultra-violet source, this being displaced from the laser 12, so that the ultra-violet rays are concentrated onto the same area on the samples or electrodes as of the laser rays d.

It is within the scope of the present invention to use other, i.e. electron beam for the irradiation of samples or electrodes in an ion source and also the use of any combinations of the above described irradiation sources.

What I claim is:

1. Anion source comprising,

means defining an enclosed fluid-tight volume having an excitation chamber for containing a substance to be ionized for analysis, an analyzing chamber for receiving a stream of ions produced upon excitation of said substance and means for transmitting said stream of ions'from said excitation chamber to said analyzing chamber,

a source of high intensity coherent radiant energy located outside said volume defining means,

said volume defining means including a transmissive portion between said source and said excitation chamber that transmits rays of said radiant energy,

and means for focusing said radiant energy from said source substantially into a point in said excitation chamber for exciting a substance therein into a stream of ions characteristic thereof for transmission to said analyzing chamber.

2. An ion source in accordance with claim 1 and further comprising,

a cathode inside said excitation chamber embracing said point for being locally heated by said radiant energy.

3. An ion source in accordance with claim 1 and further comprising,

means for instantaneously exciting said source for a prescribed time interval.

4. An ion source in accordance with claim 3 and further comprising,

means inside said excitation chamber defining a pair of electrodes,

a source of potential high enough to establish a spark between said electrodes when applied therebetween,

and means for applying said potential from said source between said electrodes only during said prescribed time interval.

5. An ion source in accordance with claim 1 wherein said source of high intensity coherent radiant energy comprises a laser.

6. An ion source in accordance with claim 5 wherein said means for focusing and said transmissive portion comprise an optical lens.

References Cited by the Examiner UNITED STATES PATENTS 3/1961 Robinson 250-413 7/1962 Fleming 250- 413 OTHER REFERENCES RALPH G. NILSON, Primary Examiner.

H. S. MILLER, G. E. MATTHEWS, A. R. BORCI-IELT, Assistant Examiners. 

1. AN IRON SOURCE COMPRISING, MEANS DEFINING AN ENCLOSED FLUID-TIGHT VOLUME HAVING AN EXCITATION CHAMBER FOR CONTAINING A SUBSTANCE TO BE IONIZED FOR ANALYSIS, AN ANALYZING CHAMBER FOR RECEIVING A STREAM OF IONS PRODUCED UPON EXCITATION OF SAID SUBSTANCE AND MEANS FOR TRANSMITTING SAID STREAM OF IONS FROM SAID EXCITATION CHAMBER TO SAID ANALYZING CHAMBER, A SOURCE OF HIGH INTENSITY COHERENT RADIANT ENERGY LOCATED OUTSIDE SAID VOLUME DEFINING MEANS, SAID VOLUME DEFINING MEANS INCLUDING A TRANSMISSIVE PORTION BETWEEN SAID SOURCE AND SAID EXCITATION CHAMBER THAT TRANSMITS RAYS OF SAID RADIANT ENERGY, 